Although some say it's from a potential rebound in coal demand, which is probably part of it, the companies above are also noted for strong safety records and safety measures, which gives them a much higher degree of predictability than others in the industry, or at least that's how they're increasingly being perceived by investors.

Adding the two elements together gives them a strong competitive advantage if they can continue on as they are, along with producing a lot of coal to meet the growing demand, assuming that the coal demand is sustainable.